The present invention relates to a two-piece solid golf ball excellent in resilience and thereby flight distance characteristic, and feel of hitting and durability.
Golf balls have been requited to be excellent in resilience and thereby flight performance, and feel of hitting upon shots and durability, and to meet such requirements, various kinds of golf balls have been proposed, for example, in Japanese Patent Laid-open Nos. Hei 6-198005 and Hei 8-294549, and U.S. Pat. Nos. 5,971,870, 6,152,835, and 6,218,453B1.
The use of silicone materials for producing golf balls has been proposed, for example, in Japanese Patent Laid-open No. 2001-170213, and U.S. Pat. Nos. 6,159,110, 6,162,134, and 6,204,331. Such golf balls using silicon materials, however, have been recently required by golf players to be further improved in terms of resilience, feel of hitting, and durability.
In view of the foregoing, the present invention has been made, and an object of the present invention is to provide a two-piece golf ball excellent in resilience and thereby flight distance characteristic, and further feel of hitting and durability.
To achieve the above object of the present invention, according to an aspect of the present invention, there is provided a two-piece golf ball including: a core made from a rubber composition containing a main rubber component which mainly contains polybutadiene; and a cover having a thickness of 1.0 to 3.0 mm, the cover being made from a material mainly containing an ionomer resin in which at least one kind of silicone powder selected from a silicone rubber powder, a silicon resin powder, and a composite powder thereof is dispersedly blended; wherein the core has a hardness corresponding to a deflection in a range of 3.3 to 6.0 mm under an applied load of 100 kg; the cover has a Durometer D hardness in a range of 55 to 70; and dimples of the number of 300 to 550 pieces are formed in the surface of the golf ball in such a manner that a dimple total volume ratio VR, which is a ratio of the total of volumes of dimple spaces under the planes surrounded by edges of the dimples in the surface of the gold ball to the total volume of a virtual ball as a result of assumption that no dimple is formed in the surface of the golf ball, is in a range of 0.85% or less.
An average particle size of the silicon powder is preferably in a range of 0.5 to 700 xcexcm.
The content of the silicon powder is preferably in a range of 0.5 to 20 parts by weight on the basis of 100 parts by weight of the ionomer resin.
The rubber composition forming the core preferably contains an organic sulfur compound.
The rubber composition forming the core preferably contains a filler for adjustment of a specific gravity, the filler mainly containing zinc oxide and additionally containing barium sulfate in an amount of 10 parts or less by weight on the basis of 100 parts by weight of the main rubber component.
The cover preferably contains an inorganic filler.
The inorganic filler used for the core is preferably barium sulfate.
According to the golf ball of the present invention, since the silicone powder is dispersedly blended in the cover, the resilience of the ball is improved, which compensates for a reduction in resilience due to hardening of the ball, to thereby improve the flight performance, and further the temperature dependency of the ball is improved. In addition, since the core is soft, the initial condition upon hitting can be set with a low spin rate and a high launch angle, to increase the flight distance of the ball, and the feel of hitting upon full-shot with a driver becomes very soft. Also, since the dimple total volume is optimized, it is possible to prevent occurrence of a drop phenomenon of the trajectory of the ball due to a low spin rate, and hence to obtain a good flight distance characteristic due to optimization of the trajectory of the ball. Further, since the core contains an organic sulfur compound, it is possible to improve the resilience of the core, and since the cover contains an inorganic filler, especially, barium sulfate, it is possible to improve the durability of the cover against cracking.
The present invention will be more fully described below.
The two-piece golf ball of the present invention includes a core and a cover.
The core is formed from a rubber composition containing a main rubber component. The main rubber component mainly contains polybutadiene which preferably contains 40% or more, especially, 90% or more of cis-1,4-bonds. The main rubber component may contain, in addition to polybutadiene, a diene based rubber such as polyisoprene rubber, styrene-butadiene rubber, or natural rubber. The content of polybutadiene in the main rubber component is preferably in a range of 50% (xe2x80x9c% by weightxe2x80x9d, the same applying correspondingly to the following) or more, especially, 70% or more.
The rubber composition used herein contains, in addition to the main rubber component, a crosslinking agent, preferably, in an amount of 15 to 40 parts (xe2x80x9cparts by weightxe2x80x9d, the same applying correspondingly to the following) on the basis of 100 parts of the main rubber component. The crosslinking agent may be selected from zinc salts, magnesium salts, and other metal salts of unsaturated fatty acids such as zinc acrylate and zinc methacrylate, esters such as triethanolpropane methacrylate, and unsaturated fatty acids such as methacrylic acids.
The rubber composition may also contain an organic peroxide such as dicumyl peroxide, preferably, in an amount of 0.1 to 3 parts on the basis of 100 parts of the main rubber component. To improve the resilience of the core, the rubber composition may further contain a vulcanizing agent such as an organic sulfur compound, for example, zinc salt of pentachlorothiophenol or diphenyldisulfide in an amount of 0.01 to 5 parts, especially, 0.2 to 3 parts on the basis of 100 parts of the main rubber component.
The rubber composition may further contain, if needed, an antioxidant such as 2,2-methylene bis(4-methyl-6-tert-buthylphenol), and a filler for adjustment of a specific gravity, such as zinc oxide, barium sulfate, or calcium carbonate. The filler may be generally contained in the main rubber component in an amount of 130 parts or less on the basis of 100 parts of the main rubber component. In particular, to improve the resilience of the core, the filler may be contained in the main rubber component in an amount of, preferably, 50 parts or less, more preferably, 45 parts or less, especially, 40 parts or less on the basis of 100 parts of the main rubber component. The lower limit of the content of the filler may be set to 1 part or more, especially, 3 parts or more, and further, 20 parts or more. In particular, to adjust a specific gravity, the filler composed of a combination of barium sulfate and zinc oxide is often used; however, from the viewpoint of improvement of the resilience of the core, it is preferred for the filler to mainly contain zinc oxide and additionally contain barium sulfate in a range of 10 parts or less, especially, 0 part.
The core can be prepared from the above-described rubber composition, for example, by kneading the components of the rubber composition in an ordinary kneader such as a Banbury mixer or a roll mill, and molding the resultant compound into a desired shape by a compression molding process or an injection molding process. In this molding process, vulcanization can be performed at a temperature of 130 to 180xc2x0 C. for 10 to 60 min.
The deflection (corresponding to hardness) of the core under an applied load of 100 kg may be in a range of 3.3 mm or more, preferably, 3.5 mm or more, more preferably, 3.7 mm or more, with the upper limit thereof being set to 6.0 mm or less, preferably, 5.8 mm or less, more preferably, 5.5 mm or less. Too small a deflection of the core (too large a hardness of the core) tends to give a hard feel of hitting the ball, and to increase the spin rates and thereby reduce the flight distance of the ball. Too large a deflection of the core (too small a hardness of the core) tends to reduce the resilience of the core and also the durability of the core against cracking. In addition, the diameter of the core is determined by the thickness of the cover.
The cover is made from a material mainly containing an ionomer resin. The kind of ionomer resin is, as will be described below, selected to give a specific hardness of the cover.
As a feature of the present invention, a silicone powder is dispersedly blended in the cover by dispersedly blending at least one kind of silicone powder (which is a powder of previously hardened silicone) selected from a silicone rubber powder, a silicone resin powder, and a composite powder thereof in the ionomer resin forming the cover.
An appropriate silicone rubber powder is exemplified by a fine powder having a highly polymerized three-dimensional structure, which is obtained by crosslinking linear dimethyl polysiloxane and/or methylphenyl polysiloxane added with 0.05 mole % or more of vinyl groups by using methyl hydrogen polysiloxane as a crosslinking agent, and a powder modified therefrom. In addition, the silicone rubber powder used herein is preferable to have a true specific gravity of about 0.97. Examples of such a silicone rubber powder include commercially available products such as KMP597, 598, 594, and 595 (spherical type) and X-52-875 (amorphous type) from Shin-Etsu Chemical Co., Ltd.
An appropriate silicone resin powder is exemplified by a fine powder of hardened polyorgano silsesquioxanes obtained by hardening siloxane bonds in a three-dimensional network expressed by a formula (RSiO3/2)n, and a powder modified therefrom. In the formula, it is recommended that R be CH3, C6H5, or a long-chain alkyl group. In addition, the silicone resin powder used herein is preferable to have a true specific gravity of about 1.3. Examples of such a silicone resin powder include commercially available products such as KMP590, X-52-1186, and X-52-854 (spherical type), and X-52-821, X-52-830, and X-52-831 (modified type, for example, modified with vinyl groups, epoxy groups, amino groups, and the like) produced by Shin-Etsu Chemical Co., Ltd.
An appropriate composite powder is exemplified by a powder obtained by covering the above silicone rubber powder with the above silicone resin, and a powder modified therefrom. In addition, the silicone composite powder used herein is preferable to have a true specific gravity of 1.0 to 0.98. Examples of such a silicone composite powder include commercially available products such as KMP600 and X-52-1139G produced by Shin-Etsu Chemical Co., Ltd.
The silicone powder used herein may be in the form of either amorphous particles or spherical particles, although the silicone powder in the form of spherical particles is preferred.
Either of the silicone powders is recommended to have an average particle size in a range of, generally, 0.5 xcexcm or more, preferably, 1 xcexcm or more, more preferably, 3 xcexcm or more, with the upper limit thereof being set to 700 xcexcm or less, preferably, 500 xcexcm or less, more preferably, 100 xcexcm or less. Too small an average particle size of the silicone powder tends to cause a large amount of scattering of the powder in the dispersion step, which is undesirable for production of the cover, whereas too large an average particle size of the silicone powder tends to degrade scattering of the silicon powder and thereby reduce the durability of the cover against repetitive shots.
The content of the silicone powder may be in a range of 0.5 part or more, preferably, 0.8 part or more, more preferably, 1 part or more on the basis of 100 parts of the ionomer resin, with the upper limit thereof being set to 20 parts or less, preferably, 15 parts or less, more preferably, 8 parts or less on the basis of 100 parts of the ionomer resin. Too small a content of the silicone powder fails to sufficiently achieve the effect of adding the silicone powder as the feature of the present invention, whereas too large a content of the silicone powder tends to cause a difficulty in uniform dispersion of the silicone powder, and thereby reduce the resilience of the cover and the durability of the cover.
The ionomer resin may further contain an inorganic filler. This is effective to improve the durability of the ball against cracking. A preferred inorganic filler is barium sulfate. The content of the inorganic filler may be in a range of 3 to 30 parts, especially, 5 to 25 parts on the basis of 100 parts by the ionomer resin.
The Durometer D hardness of the cover, which is measured under JIS K-7215, may be in a range of 55 or more, preferably, 56 or more, more preferably, 58 or more, with the upper limit thereof being set to 70 or less, preferably, 67 or less, more preferably, 65 or less. Too small a hardness of the cover tends to reduce the resilience of the cover, whereas too large a hardness of the cover tends to give a hard feel of hitting the ball, and reduce the durability of the cover against cracking.
The hardness of the cover is the value obtained by measuring the hardness of a sheet formed from the same material as that of the cover under JIS K-7215.
The thickness of the cover may be in a range of 1.0 mm or more, preferably, 1.2 mm or more, more preferably, 1.5 mm or more, with the upper limit thereof being set to 3.0 mm or less, preferably, 2.5 mm or less, more preferably, 2.3 mm or less. Too small a thickness of the cover tends to reduce the durability of the cover against cracking, whereas too large a thickness of the cover tends to give a hard feel of hitting the ball.
The two-piece golf ball of the present invention can be produced in accordance with a known method, and can be subjected to polishing, painting, and the like after formation of the cover in accordance with a known method.
Like conventional golf balls, the two-piece golf ball of the present invention has, on its surface, a multiplicity of dimples. The number of the dimples may be in a range of 300 or more, preferably, 320 or more, more preferably, 340 or more, with the upper limit thereof being set to 550 or less, preferably, 520 or less, more preferably, 500 or less. Too small the number of the dimples fails to ensure a sufficient total volume of the dimples, whereas too large the number of the dimples reduces the size of each dimple, which is liable to be affected by the paint to be applied on the surface of the golf ball, thereby failing to sufficiently achieve the ball characteristics.
With respect to the dimples formed in the surface of the golf ball, a dimple total volume ratio VR, which is a ratio of the total of volumes of dimple spaces under the planes surrounded by edges of the dimples in the surface of the gold ball to the total volume of a virtual ball as a result of assumption that no dimple is formed in the surface of the golf ball, may be in a range of 0.85% or less, preferably, 0.84% or less, more preferably, 0.83% or less, with the lower limit thereof being set to 0.60% or more, especially, 0.65% or more. Too large the simple total volume ratio VR tends to make the trajectory of the ball lower, which may cause a drop phenomenon as a result of combination of the reduced spin rate, thereby reducing the flight distance of the ball, whereas too small the dimple total volume ratio VR tends to make the trajectory of the ball higher, which may cause an inconvenience that the flight distance of the ball be reduced and also the ball be liable to be affected by wind.
The two-piece golf ball of the present invention may be formed in accordance with the Rules of Golf, that is, with the diameter set to 42.67 mm or more and the weight set to 45.93 g or less.
The two-piece golf ball of the present invention configured as described above is excellent in resilience, flight distance characteristic, durability, and feel of hitting.